The invention relates to a process for operating an internal combustion engine.
A customary .lambda. adjustment adjusts the mixture of fuel and air to be fed to an internal combustion engine to a stoichiometric ratio. During special operating conditions which require a rich mixture, the .lambda. adjustment must therefore be switched off and its task is assumed by a control.
This process works satisfactorily as long as the control sets the required rich mixture correctly during the special operation. However, maladjustment or corresponding long-term changes may lead to the setting of a lean mixture instead of the required rich mixture. Particularly towards the end of a special operation, when the rich mixture is brought back to a stoichiometric mixture ratio in order to achieve a continuous transition to the subsequent .lambda. adjustment, even small maladjustments of the control in the lean direction lead to an undesirably lean mixture. Since, during control, there is no feedback, this error also remains undetected and manifests itself only in a poorer operating behavior of the engine.
U.S. Pat. No. 4,753,209 discloses a mixture adjustment system for an internal combustion engine with a .lambda. adjustment, the .lambda. probe supplying a linear output signal. Prior to the readiness of the .lambda. probe for operation, temperature-dependent control of a choke valve is carried out. During the warm-up phase of the engine and after the operating temperature of the .lambda. probe has been achieved, coarse .lambda. adjustment takes place via the choke valve and fine .lambda. adjustment via a bypass valve. The use of a .lambda. probe with a linear characteristic ensures that a fuel/air mixture in a range from lean to rich can be set even in the warm-up phase of the internal combustion engine.